1. Field of The Invention
The present invention relates generally to an active suspension system for an automotive vehicle. More particularly, the invention relates to an active suspension system including a hydraulic circuit which prevents vehicle height from varying suddenly upon initiation of suspension control operation.
2. Description of The Background Art
U.S. Pat. No. 4,865,348, issued on Sept. 12, 1989, entitled "ACTIVELY CONTROLLED AUTOMOTIVE SUSPENSION SYSTEM WITH LINE PRESSURE CONTROL UNDER LOW FLUID SOURCE PRESSURE", assigned to Nissan Motor Company, Limited and Kayaba Kogyo Kabushiki Kaisha, discloses one typical construction of an active suspension system for a vehicle. This system includes generally a check valve disposed in a supply line between a hydraulic power source and a pressure control valve, a shut-off valve replaced in a return line, and a relief valve arranged parallel to the shut-off valve. The shut-off valve is responsive to power source pressure lower than a preselected pressure level to block fluid flow therethrough for maintaining pressure in a hydraulic circuit downstream of the shut-off valve at a constant level or neutral pressure. This prevents a vehicle body from being lowered suddenly when suspension control operation is stopped.
However, due to leakage of working fluid, the pressure held in the hydraulic circuit tends to drop gradually, resulting in pressure being lowered from the neutral pressure. Therefore, if an engine is started, actuating a hydraulic power source unit to discharge pressurized working fluid after the pressure drops in the hydraulic circuit, working fluid is fed to the pressure control valve quickly when the discharge pressure level exceeds the internal pressure of the hydraulic circuit, resulting in excessive increase in line pressure of the hydraulic cylinder, thus causing the vehicle body to rise suddenly. This gives passengers an unpleasant feeling.